Wiktionary
you don't say
interj. (context idiomatic English) really?; no kidding!; is that so? (context sometimes used sarcastically in response to the obvious English)
Usage examples of "you don't say".
But until we know for a certainty, I'd appreciate if you don't say anything to anyone about it.
If you don't say where it is, the guillotine is outside and I will be your executioner.